A distributed feedback (DFB) laser is a type of laser diode, quantum cascade laser or optical fiber laser where the active region of the device is periodically structured as a diffraction grating. The diffraction grating acts as the wavelength selective element for at least one of the minors and provides the feedback, reflecting light back into the cavity to form the resonator. In the case of a semiconductor diode laser the diffraction grating includes a grating layer having a periodic refractive index which is different from the refractive index of the adjacent layers. One type of DFB laser has a step-index planar (slab) waveguide.
The DFB laser operates in a single mode emitting laser light of a stable single wavelength and thus is widely used as the light source in optical communication systems. The emission wavelength (λDFB) of the DFB laser is determined by the formula λDFB=2neffΛ, wherein Λ is the spatial period of the diffraction grating and neff is the effective refractive index of the waveguide of the laser device. Thus, λDFB can be determined independently of the peak wavelength of the optical gain of the active layer. The DFB laser is categorized into two types including a refractive-index coupling type and a gain-coupling type based on the material of the diffraction grating.
High power C-band (1530 nm to 1565 nm) operating wavelength range DFB laser chips are available with ex-facet powers ˜200 mW, slope efficiency ˜0.2 W/A and optical far field with a 2:1 vertical to horizontal aspect ratio. These devices have relative intensity noise (RIN) values of ˜−155 dB/Hz and native linewidths of ˜500 kHz. While such DFB laser chips are generally useful, performance improvements are needed. Specifically, higher output power with better slope efficiency is needed together with a far field along with an aspect ratio closer to 1:1 to allow better coupling efficiency into circular waveguides such as optical fibers. Also, the RIN and linewidth provided by conventional DFB lasers operated at higher power can result in unacceptably high amplitude and phase noise for some applications.